MONTREAL, July 5 ? As the mercury-containing preservative thimerosal was removed from vaccines, and as fewer children received the mumps-measles-rubella vaccine, the rates of autism and related disorders rose among Canadian school children.
In a study of nearly 28,000 children born between 1987 and 1998, the prevalence of pervasive developmental disorders was greater in those children vaccinated after the mercury-containing compound thimerosal was completely eliminated from vaccines in Canada, reported Eric Fombonne, M.D., of McGill University in Montreal, and colleagues.
Similarly, there was in increase in pervasive developmental disorders prevalence even as MMR vaccination rates declined, and the rate of increase remained constant even after a second dose of the vaccine was added to the recommended schedule, the investigators reported in the July issue of Pediatrics.
"In the past, concern about a potential link between MMR vaccinations and autism led some parents to take the drastic step of refusing to inoculate their children against dangerous childhood diseases like measles," Dr. Fombonne said. "This action resulted in resurgence of the measles, which caused the deaths of several young children in Europe. We hope this study will finally put to rest the pervasive belief linking vaccines with developmental diseases like autism."
In 2004, the Institute of Medicine committee on vaccine safety concluded that "the evidence favors rejection of a causal relationship between 1) thimerosal-containing vaccines and autism, and 2) MMR vaccine and autism."
Other studies have failed to find evidence of either a thimerosal- pervasive developmental disorders or MMR vaccine- pervasive developmental disorders link. For example, when thimerosal was removed from vaccines in Sweden and Denmark, there was no corresponding change in rates of autism reported in those countries.
In addition, there has been no evidence of autism onset clustering around MMR vaccinations, and in studies in Denmark where all children are tracked, there was no evidence of a link between MMR vaccination and autism in regard to age at vaccination, time of vaccination, or years of vaccination, according to Walter A. Orenstein, M.D., director of vaccine and policy development at Emory in Atlanta.
"Despite the accumulation of negative studies, concerns from the public have not been entirely alleviated, and fears continue to be fueled by well-publicized media accounts of a spectacular nature," Dr. Fombonne and colleagues wrote.
"Unfortunately, these unsubstantiated claims have led to the uncontrolled development of chelation therapies of autistic children in North America," they added. "These therapies are not only of unproven efficacy, but they also can be dangerous, as unfortunately shown in the recent death of a five-year-old boy with autism."
Dr. Fombonne and colleagues at Montreal Children's Hospital and the Lester B. Pearson School board in Montreal performed a surveillance study to estimate the prevalence of pervasive developmental disorders in Montreal in children born from 1987 to 1998.
They also planned to evaluate the relationship of trends in pervasive developmental disorders rates with changes in cumulative exposure to ethylmercury (the form of the element found in thimerosal) that occurred due to changes in immunization schedule, as well as changes in MMR vaccination rates, and increased MMR vaccine exposure due to the introduction of a second dose during the study period.
They tracked 27,749 children born between 1987 to 1998 who were attending a total of 55 English-speaking schools in Montreal. The cumulative exposure to thimerosal by age two years for each child in each of three time cohorts was calculated.
The authors found that ethylmercury exposure ranged from medium (100-125 mcg) in the children born between from 1987 and 1991, high (200-225 mcg) in those who were born between 1992 and 1995, and "nil" from 1996 onwards, when thimerosal was entirely removed from vaccines used in Canada.
They also estimated MMR coverage for each birth cohort based on surveys of vaccination rates from 1987 to 1995, during which time children received a single vaccine dose at 12 months of age, and from 1996 to 1998, following the introduction of a second dose of vaccine at age 18 months.
They found a total of 180 children (82.8% boys) who were judged to have a pervasive developmental disorder by a special needs team. The prevalence of pervasive developmental disorders was 64.9 per 10,000. The prevalence for specific pervasive developmental disorder subtypes were:
- Austistic disorder: 21.6 /10,000
- Pervasive developmental disorder not otherwise specified 32.8/10,000
- Asperger syndrome: 10.1/10,000.
The authors saw a statistically significant linear increase in pervasive developmental disorders during the study, but the prevalence was significantly higher among those children who received thimerosal-free vaccines. There was a significantly (OR: 1.39; 95% CI: 1.01-1.92; P